Will Allah accept the repentance of one who jokingly said that he would apostatize, and will his previous deeds be invalidated because of this word, and what must he do?

For a person to say of themselves that they will disbelieve is an act of disbelief. Repentance erases sin, no matter how great. As long as you have repented and regretted your sin, its sinfulness has been removed, and one who repents from sin is like one who has no sin. So, praise Allah for repentance, resolve not to return to such a sin, watch yourself, and be careful about what you utter, for most of the sins of the son of Adam are in his tongue.